1. Field of the Invention
The present invention relates to a CRT (cathode-ray tube) correcting circuit for correcting the misconvergence of electron beams on a raster formed on a CRT and for correcting quadrupole distortion.
2. Description of the Prior Art
The quality of a picture displayed on a color cathode-ray tube (hereinafter abbreviated to "CRT") having three electron guns and a fluorescent screen which is scanned with electron beams emitted by the three electron guns deteriorates with increase in the deflection angle or in the size of the fluorescent screen due to the misconvergence of the electron beams and the distortion of the beam spots.
FIG. 5 shows an example of the vertical misconvergence of electron beams. Generally, the mislanding of three electron beams, i.e., a red electron beam R, a green electron beam G and a blue electron beam B, occurs in the four corners of a monitor screen to cause color misconvergence in the four corners. Hereinafter, such misconvergence will be referred to as X-misconvergence.
FIG. 6 shows the distortion of the beam spot of an electron beam in the four corners of a monitor screen. Such distortion of the beam spot of an electron beam deteriorates resolution.
Such X-misconvergence in the four corners of a monitor screen, in general, can be corrected by providing a correcting deflection coil on the neck of the CRT and supplying a saw-tooth signal as shown by continuous lines in FIG. 7 to the correcting deflection coil. However, since the upper and lower halves differ from each other in the direction of deviation of the electron beams for the X-misconvergence as shown in FIG. 5, a correction signal as indicated by dotted lines in FIG. 7 must also be supplied to the correcting deflection coil. Accordingly, in correcting the X-misconvergence or quadrupole distortion correction signals of opposite polarities must be used respectively for the upper and lower halves of the monitor screen.
Such correction signals of opposite polarities are produced by a system which amplifies saw-tooth signals by an analog amplifier and subjects the amplified saw-tooth signals to amplitude modulation or by a system as shown in FIG. 8 previously proposed by the applicant of the present patent application which supplies a current from a power source e.sub.AC through a choke coil ch to a correcting deflection coil Lc and turns on and off the coil Lc and a resonance capacitor Cc by switching diodes D.sub.1 to D.sub.4. However, the former system consumes a lot of power and needs a complex device comprising a comparatively large number of parts. Although the latter system is able to operate at a high power efficiency, it is difficult to determine the switching characteristics of the transistor Q for driving the diodes D.sub.1 to D.sub.4, and the system is unable to provide a satisfactory correcting signal and is expensive because the system needs transformer T.